The present invention relates generally to electronic filters for processing communication signals, and more particularly to an electronic filter called a step attenuator and a circuit modification for improving the insertion loss of the step attenuator.
A step attenuator is a filter used primarily (but not exclusively) in a bi-directional cable television (or CATV) network. The step attenuator is also bi-directional. It processes signals transmitted from a headend to a subscriber's home (i.e., “forward path”) and also processes signals transmitted from the subscriber's home to the headend (i.e., “return path”). Usually, the forward path signals are transmitted in a higher frequency band than the return path signals. In the United States, for example, the forward path frequency band is 54-1000 MHz and the return path band is 5-42 MHz. A step attenuator is usually installed in the CATV network at a subscriber tap port, just upstream of the subscriber drop cable. Step attenuators are used to equalize signal levels in the return path (and, at the same, time reduce ingress) by introducing graduated amounts (i.e., steps) of attenuation at different points in the CATV network. Step attenuators attenuate signals in the return path by a specified amount, while not disturbing the forward path. A step attenuator (and its operation) are further described in U.S. Pat. No. 5,745,838.
In FIG. 1 herein, a schematic of a prior art step attenuator circuit is shown. A step attenuator 10 includes a housing 12, an input connector 14, an output connector 16, an input conductor 18, and an output conductor 20. Step attenuator 10 includes a highpass filter network 22 connected in parallel with a lowpass filter network 24. Lowpass network 24 includes a resistive attenuator network 26. Highpass network 22 is configured to pass high frequency signals (i.e., in the forward path frequency band) and suppress low frequency signals (i.e., signals in the return path frequency band). Lowpass network 24 is configured to pass the low frequency signals and suppress the high frequency signals. Attenuator network 26 is configured to attenuate the low frequency signals by a specified amount as the signals pass through lowpass network 24. The low frequency signals are attenuated by network 26 independently of the high frequency signals.
Prior art step attenuator 10 has functioned very well in its intended environment for over ten years. In recent years, the high frequency (or forward path) spectrum of CATV networks has been extended. It now extends to 1000 MHz, and is anticipated to go even higher, possibly up to 3000 MHz. As signal frequencies approach 1000 MHz and higher, the parasitic capacitance in a step attenuator circuit begins to adversely affect performance. In particular, the parasitic capacitance between highpass network 22 and attenuator network 26 causes leakage of high frequency signal currents (or “leakage currents”) from network 22 to network 26 where the power or energy of the leakage currents is absorbed (i.e., lost) in the resistive or lossy elements of network 26 (i.e., I2R loss). These leakage currents establish high frequency voltages across R3 and R1 (FIG. 1). This lost power manifests itself in the form of a higher insertion loss for the step attenuator, and is particularly noticeable at ultra high frequencies (UHF) above 750 MHz.
Lowpass network 24 is implemented as an elliptic-function lowpass filter, as indicated by the characteristic inductor/capacitor shunt branches, C6/L4 and C7/L5 in FIG. 1. In an elliptic-function filter, inductors in the shunt branches (L4 & L5) present a high shunt reactance to any of the high frequency signals that reach them. The leakage currents of the high frequency signals (from highpass network 22) will be substantially reflected by the L/C shunt branches (and also by series inductors L3 & L6) in lowpass network 124, and will thus flow through resistors R1-R3 of attenuator network 26 (FIG. 1). This concentration of leakage currents in network 26 provides a worse case for the I2R power loss of the high frequency signals. Thus, an elliptic-function implementation of lowpass network 24 is less than ideal in a step attenuator circuit, if the only goal is to minimize the adverse effects of parasitic capacitance. However, an elliptic-function lowpass network is usually chosen for a step attenuator, because it provides a sharper transition region than most implementations for a given number of electrical components used (i.e., it is an efficient design) and it provides a relatively flat passband response while still achieving a good transition region (usually a tradeoff). Thus, rather than switch to an implementation that has only a capacitor or capacitors in the shunt branch (providing a lower reactance to the high frequency signals), e.g., as shown in FIG. 1 of U.S. Pat. No. 6,784,760, it would be more desirable to find another solution to minimize the effects of parasitic capacitance.
FIG. 2 is a simulated plot of the frequency response for step attenuator 10. As shown, a low frequency response 32 (i.e., 5-42 MHz) is attenuated about 6 dB by attenuator network 26. A high frequency response 34 (i.e., 54-1000 MHz) exhibits little attenuation over a broad range of frequencies, but a noticeable amount of insertion loss 36 (or attenuation) appears above 750 MHz. As explained above, insertion loss 36 is caused (at least in significant part) by the parasitic capacitance between highpass network 22 and attenuator network 26. Heretofore, insertion loss 36 has been tolerable. However, as television service extends to 1000 MHz and above, insertion loss 36 becomes less and less tolerable, especially when additional equipment is being used in the subscriber's home (which is becoming more and more the case). A solution to correct or improve (i.e., reduce) the insertion loss is needed.
Known techniques to minimize parasitic capacitance in high frequency circuits have included (1) careful layout of circuit components and traces on the printed circuit board (PCB), (2) ample separation of the components and traces, and (3) the use of shielding, ground planes, and/or terminations where possible. Certainly, the technique of carefully laying out components and traces has been employed with success in step attenuators and like filters. However, as shown in FIG. 2, a noticeable amount of insertion loss (36) still remains at the ultra high frequencies above 750 MHz. Step attenuators are designed to be very small and compact so that they can be conveniently installed at the subscriber tap ports. Thus, there is little room for separating circuit components and board traces in current step attenuator packages.
To achieve compact size, step attenuator circuits are usually mounted on both sides of a small circuit board. If metal shields are used to minimize parasitic capacitance, all of the circuit components may need to be relocated to one side of the circuit board, thus requiring a significantly larger board. Thus, the shielding technique may not be an optimum solution, and would likely add to the cost of manufacturer. Step attenuators need to be low cost devices, because a great number of them are usually deployed throughout a communication network. The use of ground planes (e.g., sandwiched between layers of printed circuit board) to reduce parasitic capacitance may also add to the cost and complexity of manufacture and produce unintended stray capacitance, resulting in frequency response roll-off. Thus, a more optimum solution to reduce the effects of parasitic capacitance is needed.